Thermoplastic resin microporous membranes have been widely used, for example, as a material for separation, selective permeation, and isolation of substances: such as separators for batteries used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries; separators for electric double layer capacitors; various filters such as a reverse osmosis filtration membrane, an ultrafiltration membrane, and a microfiltration membrane; moisture-permeable waterproof clothing; and medical materials. In particular, a polyethylene porous membrane has been suitably used as a separator for lithium ion secondary batteries, because such a porous membrane has ion permeability when impregnated with an electrolyte; is excellent in electrical insulation, electrolyte resistance, and oxidation resistance; and has such a pore-blocking effect that, at the time of abnormal temperature rise in a battery, an excessive temperature rise is suppressed by blocking a current at a temperature of approximately 120° C. to 150° C. However, when the temperature rise continues for some reason even after the pore blocking, membrane rupture sometimes occurs due to a decrease in the viscosity of a polyethylene constituting the membrane and in the shrinkage of the membrane. This phenomenon is not limited to a phenomenon that occurs when polyethylene is used. Also in the case of using other thermoplastic resins, this phenomenon is unavoidable at a temperature equal to or higher than the melting points of the resins.
Separators for lithium ion batteries are highly responsible for battery properties, battery productivity, and battery safety, and required to have, for example, excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking properties (shutdown properties), and melt rupture properties (meltdown properties). Furthermore, the separators are required to have improved adhesiveness to an electrode material for the purpose of improving cycle characteristics of a battery and required to have improved electrolyte permeability for the purpose of improving productivity. Therefore, various studies to laminate various modified porous layers to a porous membrane have been conducted until now. For the modified porous layer, polyamide-imide resin, polyimide resin, or aromatic polyamide resin, each having both heat resistance and electrolyte permeability, and fluororesin excellent in adhesiveness to electrodes are suitably used. Incidentally, the “modified porous layer” used herein means a porous layer that has at least adhesiveness to an electrode material and the effect of inhibiting thermal shrinkage (heat resistance).
Fluororesin is a resin relatively excellent in adhesiveness to electrodes, but, a porous layer formed of a fluororesin has higher thermal shrinkage than porous layers formed of a polyamide imide resin, a polyimide resin, and an aromatic polyamide resin, and has a lower level of safety when incorporated into a lithium ion battery. To improve such heat resistance, a method of adding inorganic particles or organic particles has been proposed, but, such a method causes a decrease in an important required property, namely, adhesiveness to electrodes. In other words, it has been extremely difficult to achieve both adhesiveness to electrodes and heat resistance.
Furthermore, in a battery that is expected to be used for electric automobiles and the like, which require the battery to withstand severe operating conditions, it is expected that not only the safety is increasingly ensured, but also, with lower costs and higher capacity, a separator is further made thinner to increase an area capable of being filled in a container of the battery.
Examples of Patent Literature 1 disclose an organic-inorganic composite porous separator membrane having thermal stability and excellent ion permeability. The separator membrane is obtained by applying an acetone solution of a mixture of inorganic particles and a binder formed of a copolymer including polyvinylidene fluoride (inorganic particles/binder=90/10 (% by weight) onto a polyethylene separator membrane.
Patent Literature 2 discloses a separator for nonaqueous secondary batteries in which an adhesive porous layer formed in at least one side of a porous base material, formed of a polyvinylidene fluoride resin, and having a crystallinity of 20% to 35% is laminated. In examples of Patent Literature 2, a separator for nonaqueous secondary batteries including a laminated adhesive porous layer and achieving both ion permeability and adhesiveness to electrodes is obtained by applying a dimethyl acetamide/tripropylene glycol mixed solvent solution of a polyvinylidene fluoride resin to both sides of a polyethylene microporous membrane, and immersing the coated membrane in a coagulation liquid to coagulate, followed by water-washing and drying.
Patent Literature 3 discloses a separator for nonaqueous electrolyte batteries in which a heat resistant porous layer containing equal to or more than 70% by volume of heat resistant particles is formed on a surface of a resin porous membrane. Specifically, in Example 1, a mixed solution obtained by adding alumina powder: 3,000 g to a solution in which N-methyl-2-pyrrolidone (NMP): 1,000 g is dissolved in an NMP solution of PVDF (having a solid content of 15% by mass): 600 g is applied onto a polyethylene porous membrane, and dried, whereby a separator for nonaqueous electrolyte batteries with excellent dimensional stability at high temperature is obtained.
Examples of Patent Literature 4 disclose a separator formed of two laminated layers of a porous organic-inorganic composite internal layer and a porous polymer outer layer and having good adhesiveness to electrodes, the separator being obtained in such a manner that a slurry containing a mixture of inorganic particles, a binder formed of a polyvinylidene fluoride copolymer, and acetone, and a slurry including only the above-mentioned binder and acetone are applied onto a polyethylene porous membrane, and dried simultaneously.
Any of the separators disclosed in Patent Literatures 1 to 4 is a separator in which a modified porous layer including a PVDF resin having an adhesive function to electrodes, or including the PVDF resin and heat resistant particles is laminated on a polyolefin porous membrane. Generally, to improve adhesiveness to electrodes, the ratio of a resin having an adhesive function to electrodes, such as PVDF, in a modified porous layer is made higher. For example, ultimately, a modified porous layer including only the above-mentioned resin is ideally used. However, in this case, the effect of inhibiting thermal shrinkage is decreased, whereby sometimes the safety cannot be ensured when the separator is incorporated into a nonaqueous secondary battery. By contrast, when the ratio of the heat resistant particles is made higher, the effect of inhibiting thermal shrinkage is increased, but adhesiveness to electrodes is decreased. In other words, the fact is that, only with the adjustment of the two ratios, satisfactory adhesiveness to electrodes and a satisfactory effect of inhibiting thermal shrinkage cannot be fully achieved.
Furthermore, as nonaqueous secondary batteries have increased in performance in recent years, a separator is required to have higher ion permeability. However, when a modified porous layer is laminated on a polyolefin porous membrane, ion permeability with which the polyolefin porous membrane is originally equipped is inevitably deteriorated to some extent. Although there is a method of laminating a modified porous layer on a polyolefin porous membrane beforehand having low air permeation resistance, in other words, high porosity, the polyolefin porous membrane having high porosity has low mechanical strength, and therefore, such a method cannot be suitable for requirements for high-speed processing and a thinner separator, the requirements being associated with a reduction in cost and an increase in capacity which will rapidly proceed in the future.
In other words, there was not present a separator for batteries that achieves adhesiveness to electrodes and the effect of inhibiting thermal shrinkage (heat resistance) and suppresses the extent of increase in air permeation resistance due to the lamination of a modified porous layer.